The present invention relates to vibration monitoring for aircraft engines and, more particularly, to an accelerometer configuration for measurement of system vibration and indication of engine health for the aircraft engine.
Typical commercial engines have two accelerometers and an associated signal conditioning system to monitor engine health and provide an indication of vibration to the flight crew. On most engines, one of the accelerometers is positioned internally to the engine, at a bearing that is often the most sensitive location to fan imbalance. Since this sensor is internally located, if it fails, a third “alternate” external sensor is usually installed on the fan frame or other external location as a temporary replacement until the next engine teardown when the internally located accelerometer can be replaced. Although the alternate external location is generally not as sensitive to fan imbalance as the internally located sensor, it is still sensitive enough to provide an indication of engine health. The difference in the sensitivity of the location, measurable in g's of response, is typically somewhat compensated by installing an accelerometer with a more sensitive output, such as pico-coulomb of charge output per g of vibration.
In order to install the alternate accelerometer, the airline or other user needs to stock it as a spare part and maintain it in inventory. Further, to actually install the alternate accelerometer, the engine cowling must be opened, and the accelerometer bolted on, and hooked up with an electrical cable to carry the signal. A mounting pad or bracket also needs to be provided on the engine in order to mount the accelerometer. Engine fan balance coefficients are typically different with the alternate sensor, as compared to the originally internally located sensor, and this difference must be accommodated in the vibration system balance software at the time the alternate accelerometer is installed. All of these requirements are necessarily a maintenance burden.
While the first of the two original accelerometers is typically located internally at a location most sensitive to fan imbalance, the second, of the original two accelerometers is typically externally located on a casing or frame, for instance, but not limited to the Turbine Center Frame (TCF), Compressor Rear Frame (CRF), Turbine Rear Frame (TRF), Low Pressure Turbine (LPT) casing, TEC (Turbine Exhaust Case), or HPC (High Pressure Compressor) case. If this second of the two original accelerometers fails, it is simply replaced when convenient, usually within the next couple of flights, and does not require an alternate accelerometer.
It would be desirable to provide an accelerometer configuration that requires less than the currently required two production accelerometers and the third alternate accelerometer that is installed on the engine if the internal accelerometer fails.